Fiber optic gyroscopes (FOGs) have been developed for navigation and guidance applications. FOGs are solid state devices have the potential advantages of long life, no moving parts, ruggedness, light weight, low cost potential, freedom from warm-up or run-up time, and low voltage power.
Several types of FOGs that have been developed are open loop and closed loop interferometric fiber optic gyroscopes (IFOGs), respectively, and resonant fiber optic gyroscopes (RFOGs). In basic terms, an open loop IFOG typically consists of a semiconductor source whose light is divided. About half of the light goes into one end of a fiber optic coil and propagates clockwise and the remaining light goes into the other end of the coil and propagates counterclockwise around the coil. Light from the clockwise and counterclockwise beams emerging from the ends of the coil is combined and its intensity is measured by a photodetector. When the device is not rotating, the emerging light beams combine in phase for maximum intensity to produce a maximum output signal from the photodetector. Upon rotation of the IFOG, there is a resulting phase difference between the two emerging beams which creates an interference that reduces the intensity of the combined beam and thus the photodetector output signal. Such design has several drawbacks in that it has the least sensitivity near the at-rest condition and that the photodetector output does not indicate the direction of rotation. These characteristics can be improved if the phase between the clockwise and counterclockwise beams is shifted (i.e., biased) by 90 degrees. Typically, bias is introduced by an electro-optical phase modulator which varies the phase shift in a sinusoidal fashion. By demodulating the photodetector output signal at the same frequency, the demodulated output signal will have the desired maximum sensitivity near the at-rest condition. Also, the demodulated signal has opposite polarity for clockwise and counterclockwise inertial rotation, thus explicitly indicating the direction of rotation. Bias stability (i.e., the FOG's random drift rate in degrees per hour) is reasonably favorable. A major shortcoming of the open loop IFOG is that its output signal is only strictly linearly proportional to its rotation rate near zero rotation rates. The output becomes increasingly nonlinear at higher rates. Secondly, the output at higher rates becomes increasingly sensitive to the gains of various electronic amplifiers which are not operating near null in the open-loop gyro configurations. To overcome such nonlinearity and dependencies on such absolute gain stabilities, a closed loop FOG was developed. With the closed loop version, rotation of the IFOG causes a phase shift between the clockwise and the counterclockwise beams, thereby generating a signal which is applied to a transducer to cancel the shift. Thus, the IFOG operates near its most sensitive null position. Absolute accuracies in the electronics are no longer critical.
In one particular closed loop IFOG configuration, referred to as the "serrodyne" technique, the phase-shift transducer located at one end of the coil is excited with a sawtooth voltage, thereby imparting on the light a phase ramp with periodic resets. The ramp height is adjusted to be 2.pi. radians and the reset or flyback time is made to be very small compared to the period of the sawtooth; the light wave is effectively frequency shifted by frequency .DELTA.f of the sawtooth wave. Since both clockwise and counterclockwise waves traverse the phase modulator, both are frequency shifted before being interfered at the photodetector. However, one light wave experiences the frequency shift before traversing the coil and one light wave is frequency shifted after traversing the coil. Because two light waves thus traverse the coil at different frequencies there is a net phase difference between them which is used to null out the phase shift due to rotation. The required sawtooth frequency or effective frequency shift .DELTA.f necessary to produce a phase shift to counterbalance the phase shift due to rotation is a digital measure and linear measure of the rotation rate. This design increases the accuracy of the FOG since the output is a frequency which can be measured more accurately than an analog voltage in an open-loop case. The closed loop approach also improves the IFOG's dynamic range.
Another competing FOG technology is the RFOG which more closely resembles a ring laser gyroscope than an IFOG. The RFOG uses a short coil relative to the other FOG technologies, thereby reducing size, effects of thermal transient gradients, and cost. Also, the RFOG offers the greatest potential for enhanced accuracy. An IFOG uses a diode light source which is semi-coherent (i.e., broadband) whereas the RFOG utilizes a coherent (i.e., narrowband) light source. The RFOG's operation is based on the use of a fiber-optical cavity made from a few turns of optical fiber which is precisely tuned so that only certain discrete frequencies will travel through the cavity. The frequencies that can travel through the optical cavity in the clockwise and counterclockwise directions are known as the clockwise and counterclockwise resonant frequencies of the cavity. The clockwise and counterclockwise resonance frequencies are the same in the absence of rotation, but split in the presence of rotation. To measure this frequency splitting, a closed loop design is incorporated. Since the RFOG measures rotation-induced resonant frequency splitting in a fiber resonator, a frequency shifter is required to track the clockwise and the counterclockwise frequencies. In practice the only suitable frequency shifting technique using a "guided-wave" implementation is to use a phase modulator in conjunction with the serrodyne modulation technique. Serrodyne modulation has commonly been associated with baseband frequency shifting. However, baseband frequency shifting in the related art presents several problems when implemented in RFOGs. Namely, the use of both positive and negative going ramp signals are required to handle clockwise and counterclockwise rotation rates. Further, long ramp signals are required to null low rotation rates and the ramp signals needed to be nearly perfect in slope because imperfect ramps generate unwanted sideband frequency components which result in gyroscope errors.